Talc is a relatively abundant, inexpensive, highly water-insoluble, hydrophobic and generally unreactive mineral. It can be categorized as a hydroxylated magnesium silicate and represented by, inter alia, one or more of the formulas:
(Si.sub.2 O.sub.5).sub.2 Mg.sub.3 (OH).sub.2, PA1 Si.sub.8 Mg.sub.6 O.sub.20 (OH).sub.4, or PA1 Mg.sub.12 Si.sub.16 O.sub.40 (OH).sub.8, disregarding impurities, which can include inorganics such as carbonates, other magnesium silicates, ferrous iron compounds and various organic materials. Such impurities generally occur in minute amounts, but can occur in major amounts as well; certain talcs, for example, can contain major amounts of dolomite or tremolite. The impurities found in talcs will vary as to type and amount depending on the geographic source of the talc. Even in minute amounts, however, impurities may exert a significant influence on a talc's in-use performance as a filler for thermoplastic molded articles. PA1 " . . . will have a molecular weight greater than about 300; will contain one or more polar groups such as epoxide, aliphatic hydroxyl, ester, amide, ether or sulfide; and will preferably contain a non-polar organic group which makes them at least moderately compatible with the polymer, such as lauryl or stearyl";
Talc, being naturally organophilic, is highly compatible with and is easily wet by organic resins. Thus, it has come into widespread use as a filler in thermoplastic resinous compositions, including those containing polyolefins such as polyethylene, polypropylene and the like. Because of talc's thin platelet structure when finely ground, it is considered a reinforcing filler rather than an extender. Non-platy particulate mineral fillers such as calcium carbonate, on the other hand, are primarily extenders rather than reinforcing fillers. Polypropylene reinforced with talc, in particular, is widely used in appliance applications, where the color of the filled, molded resin is a major concern, and in automotive applications, especially under-the-hood automotive applications, where color is not a particular concern but increased stiffness, dimensional stability and resistance to heat distortion are of primary importance. Plastics producers prefer a talc filler which does not discolor at typical molding temperatures, particularly where the product will be decorative in nature, and which affords adequate impact strength and other tensile properties, and adequate long-term heat stability, to the molded plastic.
Two characteristics of a talc--its particle size distribution and its crude ore source--have been said to affect the properties of talc-filled polypropylene molded articles. Tests have shown that talc particle size distribution affects the stiffness or flexural modulus and tensile yield of a polypropylene molded article, with tensile yield being most affected when the talc filler's cumulative mean particle size is less than 7 .mu.m. Flexural modulus, on the other hand, has been shown to vary inversely with talc particle size. To further complicate the picture it has also been found that better long-term heat stability is generally realized with increased talc particle sizes; see Bragg et al., Plastics Engineering, Vol. 30, No. 9, pp. 30-32 (September 1974).
The apparent particle size distribution in talc from any geographic source, as measured by typical gravimetric particle size distribution methods, can be made comparable to that of talc from any other source by grinding. However, matching the aspect ratio of talc from one geographic source by grinding talc from another geographic source which has a different aspect ratio cannot easily be accomplished. Geographic source itself--or more particularly the different kinds and amounts of impurities present in talcs from different geographic sources--raises another problem which is not so easily solved. Talcs from certain geographic sources, such as Montana (Yellowstone and Beaverhead talcs, for example) and certain Australian talcs, have been found to be inferior to those from other sources, particularly California, as fillers for polypropylene. Bragg et al have shown, for example, that Montana talc-filled polypropylene is poorer in heat stability than California talc-filled polypropylene when treated at 365.degree. F. for 16 hours, no matter what the particle size, coarse or fine, of the talc used, and that to obtain equal long-term heat aging performance, a higher overall heat stabilizer level must be used in Montana talc-filled polypropylene than in California talc-filled polypropylene.
A comparison of the chemical composition of theoretical and typical commercial talcs is given in Radosta, Plastics Compounding, September/October 1979, pp. 23, 24, 26-28 and 30, at page 24 (Table I):
______________________________________ Theoretical Montana California Vermont pure talc talc talc talc ______________________________________ SiO.sub.2 % 63.5 62.5 57.4 56.2 MgO, % 31.7 30.6 27.6 30.8 CaO, % 0.3 6.2 0.4 Al.sub.2 O.sub.3, % 0.5 1.4 0.5 Fe.sub.2 O.sub.3, % 0.7 0.3 3.9 Loss on ignition, 4.8 5.4 7.1 8.2 ______________________________________
Mathur et al., in Society of Plastics Engineers Technical Papers, Vol. 25, pp. 663-667 (1979) reported on their studies of the deterioration of oven aging characteristics in heat-stabilized polypropylene moldings filled with certain high aspect ratio talcs. These authors found that "(t)he melt compounding of Montana talcs as well as Vermont talcs results in significant discoloration of molded parts, while California talcs do not discolor the matrix." No loss of mechanical properties, however, was seen to be associated with this discoloration. Mathur et al. also found that while "(t)he primary source for polypropylene coloration by Montana and Vermont talcs is not well understood, . . . it can be minimized by the use of . . . processing aids, such as calcium stearate, carbowax and amide processing lubricants."
U.S. Pat. No. 3,553,158, issued Jan. 5, 1971 to Gilfillan and mentioned in the Mathur et al. article, discloses heat-stabilized, talc-filled polypropylene molding resin compositions containing a "talc deactivating" organic polar compound, preferably an epoxide (particularly a polyepoxide), an amide, an acrylate polymer or an aliphatic polyol. According to Gilfillan, such compounds generally
see column 7, lines 41-58 in the Gilfillan patent. Carbowax 400 and Carbowax 6000 are specifically disclosed as "talc deactivating" compounds in Gilfillan's Table I, the talcs treated in the working examples were Montana talcs, and calcium stearate was included in the compositions of at least working examples 1-3.
Shimizu et al., in Japanese Kokai 75 8,098, published April 2, 1975 [Chem. Abst. 83: 98595j (1975)] disclose increasing the discoloration resistance of polypropylene homopolymer and copolymer molding compositions containing talc by adding thereto, as a "reforming agent", a polyalkylene glycol alkyl ether or polyalkylene glycol alkylphenyl ether such as a "polyalkylene glycol phenoxyether", polyethyleneglycol octylphenyl ether, polyethyleneglycol lauryl ether or polyethylene glycol/polypropylene glycol ether.
Other publications which disclose treating talc or talc-containing thermoplastic resinous compositions to improve the properties of thermoplastic resinous molded articles made from them include the following:
U.S. Pat. No. 4,116,897, issued Sept. 26, 1978 to Huszar et al., which discloses polyolefin molding compositions containing talc and a mixture of two surfactants--one of which can be alkylphenyl polyether--having different HLB values; see column 3, lines 1-17 and 67 and column 4, line 17.
Talc filler coated with at least one metallic salt of an 8-20 carbon atom-containing fatty acid, e.g., calcium stearate, is disclosed in U.S. Pat. No. 4,255,303, issued Mar. 10, 1981 to Keogh; see column 4, lines 13-32.
Mineral fillers such as talc coated with a thin layer of liquid ethylene oxide oligomer having a molecular weight of from 100-800 are shown in U.S. Pat. No. 4,411,704, issued Oct. 25, 1983 to Galeski et al.; see column 2, lines 11-20 and 23.
Transparent polypropylene food packaging materials which are easily incinerated after use and which contain talc, a polyol ester such as polyethyleneglycol monostearate or glycerol distearate, and antioxidants and stabilizers, included among which is epoxidized soybean oil, are shown in Tsunetsugu et al. Japanese Kokai 75 109,239, published August 28, 1975 [Chem. Abst. 83: 207145k (1975)].
Rusznak et al., Muanyag Gumi, Vol. 16, No. 9, pp. 257-261 [Hung. 1979; Chem. Abst. 92: 42775m (1980)] discloses isotactic polypropylene compositions containing talc, a surfactant and an "elastomeric adhesion improver".
Talc-filled polypropylene molding compositions containing oleic amide as a processing aid together with BHT (butylated hydroxytoluene, an antioxidant) are taught in Tokuyama Soda's Japanese Kokai 80 142,039, published Nov. 6 1980 [Chem. Abst. 94: 122583d (1981)].
The treatment of talc with solid resins, such as hydrogenated petroleum resin, wax (e.g., stearic acid) or a combination of such substances to improve the talc's compatibility with polyolefins is shown in Matsumoto et al. Japanese Kokai 78 65,346, published June 10, 1978 [Chem. Abst. 89: 147656q (1978)].
Goel et al., Polym. Eng. Sci., Vol. 20, No. 3, pp. 198-201 (1980) [Chem. Abst. 92: 129772g (1980)], disclose adding small amounts of oligomeric polypropylene oxide to talc-filled isotactic polypropylene to decrease both viscosity and elasticity.
A 1978 Research Disclosure, Vol. 173, No. 19 [Chem. Abst. 89: 180758x (1978)] teaches that low melting cellulose acetate butyrate is a good heat stabilizer for talc-filled polypropylene.
It has now been discovered that Montana talc and like talcs regarded, in the unbeneficiated state, as inferior fillers for thermoplastic resinous compositions, and particularly polypropylene molding resins, because they discolor the resinous matrix when subjected to typical molding conditions and adversely affect its long-term heat stability, can be rendered suitable for this use, at low cost, by treatment with novel combinations of particular amounts of certain materials. Thermoplastic resinous molded articles containing the thus-treated talcs exhibit significantly reduced darkening was measured by the General Electric brightness test), without unacceptably increased yellowing (as measured by the Hunterlab yellowness index), and improved heat stability (as measured by oven aging) when compared to molded articles containing the corresponding untreated talc.
It is therefore an object of this invention to provide novel, improved, low cost beneficiated talcs and compositions which provide such beneficiation.
A further object of this invention is to provide low cost means for improving the performance characteristics of talcs hitherto considered unsuitable per se as fillers for thermoplastic resinous compositions.
A still further object of this invention is to provide thermoplastic resinous compositions filled with incompatible or unsuitable talcs, such as Montana talc and like talcs, which have been improved in accordance with the invention so that articles molded therefrom exhibit physical properties, such as color, impact strength and other tensile properties, and long-term heat stability, comparable to those found in molded articles filled with "superior" talcs.
Another object of this invention is to provide talcs which have been beneficiated at low cost and which, when incorporated as fillers in thermoplastic molded articles, result in such articles exhibiting significantly reduced darkening, without unacceptably increased yellowing, comparable tensile properties and improved heat stability when compared to molded articles containing unbeneficiated talc.
These and other objects, as well as the nature, scope and utilization of this invention, will become readily apparent to those skilled in the art from the following description, the drawings and the appended claims.